Application of non-degradable waste as building material for low-cost housing

Building material is one of the essential aspects in accommodating the supply and demand of low-cost housing in Indonesia. Recently, several researchers have devoted much time and effort to developing waste recycling for building materials since it is more ecologically benign, particularly for non-degradable waste. This article focuses on recycling disposable diaper waste as composite material for a structural and architectural component of the building based on Indonesian building standards. In addition to offering a broad perspective on the implementation of experimental findings, the design scenario comprised the construction of low-cost housing with a floorplan area of 36 m2. The experimental results indicate that disposable diapers waste to use as composite materials of the building has a maximum capacity of 10% for structural components and 40% for nonstructural and architectural components. The prototype housing also reveals that 1.73 m3 of disposable diaper waste can be decreased and utilised for a housing area of 36 m2.

www.nature.com/scientificreports/ utilising waste from building materials 10,[19][20][21] , such as steel and rubber and industrial materials 10,14,16,22 , such as fly ash become the best choice to lessen the impact on the environment and costs. Other ways of recycling nonbiodegradable waste 15,23,24 , such as plastics, are considered low-cost materials. Furthermore, some researchers have developed user-friendly material that is accessible through a variety of inventions and technics, including compressed earth blocks [25][26][27][28][29] , dome construction [30][31][32] , rammed earth [33][34][35] , and vault construction [36][37][38] . Some even relate the advanced technologies 39 (e.g. intelligent construction site, simulation and modelling, digitalisation and virtualisation) to be used in construction sites and involving private organisation participation in infrastructure development 40 . Like other developing countries, low-cost housing provision in Indonesia has been a serious concern in the last three decades as the urban population has grown at a pace of 4.1% per year, and it is predicted that 68% of Indonesians will live in urban areas by 2025 41 . The benefits of urbanisation are limited 41 because of the problems that lead to more poverty, like the rise of slums because there are not enough affordable places to live 3,42 . The significant increase in the number of people living in cities has yet to consider how much land is available in cities, which has caused housing demand and land prices to go up. Due to the rapid urban population, Indonesia faces two significant consequences: housing demand and waste management.
In terms of housing demand, Indonesia has a large gap between supply and demand, with a demand for 780,000 units of housing per year and the capability of stakeholders to deliver 400,000-500,000 units per year [43][44][45] . A backlog of around 300,000 housing units every year must be resolved to provide homes for approximately 30% of urban residents who live in non-owned housing. Following government programs, housing provision is crucial, but building materials are limited. In Indonesia, concrete, bricks, wood, and ceramics continue to be the most used construction materials 46 due to their large capacity and as mandated by the building rules and regulations. Nevertheless, regarding environmental considerations, those materials create new issues, such as clay bricks and tiles having the highest embodied energy 46 , carbon emissions, and eco-costs 47 .
Furthermore, population growth is accompanied by increased waste capacity in terms of waste management. According to statistics 48 , the total waste per year in 2019 was 29.21 million tons, which rose to 32.76 million tons in 2020. Due to the situation, the Indonesian government is focusing more on waste capacity management, decreasing roughly 17.68 million tons of waste in 2021. Population growth also causes an increase in the use of disposable diapers for baby care. Since its introduction in the 1960s, the popularity of disposable diapers has risen due to the benefits of the circular economy within various diaper versions that have been adapted for more comprehensive applications over time 49 . In addition, there is a social benefit, especially for parents, as performances are convenient and affordable.
For this reason, previous studies investigated using materials innovation from disposable diaper waste as composite materials. Unfortunately, recycling this waste is now restricted to scientific research. The general population does not understand it well, but according to studies, this material innovation has considerable benefits in structural strength, economy, and environment [50][51][52][53][54][55] . The research also demonstrated that the mechanical properties and microbial content of disposable diaper concrete, in specific compositions, are identical to conventional concrete 52,53,55 . Adding 1% diaper to concrete enhances internal curing hydration and produces the most robust, durable material 52 . In addition, a mixture of up to 5% disposable diapers with concrete had the maximum strength at 28 days compared to other percentages 53 .
Furthermore, sodium chloride can sanitise used disposable diapers from a health perspective 54 . Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) tests on concrete made from disposable diapers revealed minor differences with the inclusion of clean diapers 55 . Moreover, compared to other waste management methods such as incineration and co-firing, the recycling of disposable diapers as concrete components has more significant benefits regarding carbon emissions and eco-costs 51 .
As a result, the study intends to tackle the problem of housing provision by creating building materials from non-degradable waste, which is cost-effective while meeting building standards. This study was researched to get a fresh perspective on waste usage for building components considered low-cost housing components, and recycling disposable diaper waste is recommended. The research was then carried out through an experimental examination of composite materials of building components using disposable diapers as composite materials. The project is designed to capture the material use potential based on construction standards. The Indonesian Building Standard (Standar Nasional Indonesia/SNI) has become a standard regarding material mechanical and physical qualities.

Methods
A laboratory experiment was undertaken to conduct a direct inquiry and To calculate the number of disposable diapers that can be used as construction components. The experimental study involved two kinds of composite materials, concrete composite for structural elements such as columns and beam thus mortar composite for architectural elements such as non-bearing walls and floors. The standard of building materials following Indonesian building standards and regulations (Standar Nasional Indonesia/SNI) as follows: www.nature.com/scientificreports/ Identifying the calculation formula for composite materials. When changing or substituting composite materials in a concrete component, the difference in material densities must be considered. For instance, the method does not directly measure weight capacity by percentage when fine aggregate is substituted with disposable diapers. The apparent similarity between 300 g of fine aggregate and 300 g of disposable diapers is deceiving. Because disposable diapers are lighter than fine aggregates, 300 g occupy a much larger volume. It is necessary to convert the weight of materials using their densities to balance the capacity. The following formula is then used to obtain the maximum replacement capacity: wherem w = mass of waste material capacity (g),ρ w = density of waste materials (g/cm 3 ),ρ fa = density of fine aggregate (g/cm 3 ),m fa = mass of fine aggregate (g). Consequently, the formula compromises as follows by adding the percentage of recyclable materials: whereR wc = capacity of recycled waste materials (g),ρ w = density of waste materials (g/cm 3 ),ρ fa = density of fine aggregate (g/cm 3 ),m fa = mass of fine aggregate (g),% rep = replacement percentage of fine aggregate by waste material (%). In this study, waste material is repurposed into disposable diapers, and the formula is created to: wherem d = mass of disposable waste diaper (g),ρ d = density of disposable waste diaper (g/cm 3 ),ρ fa = density of fine aggregate (g/cm 3 ),m fa = mass of fine aggregate (g).
In consequence, the formula compromises as follows by including a percentage of recyclable disposable diapers: whereR dc = capacity of a recycled disposable diaper (g),ρ d = density of disposable waste diaper (g/cm 3 ),ρ fa = density of fine aggregate (g/cm 3 ),m fa = mass of fine aggregate (g).
Determining standards for building materials. In order to be used as construction materials, the experiment's results must fulfil the appropriate building material criteria. The use and specifications for concrete based on compressive strength are displayed in Table 1. The standard distinguishes between three strengths of concrete, ranging from low to high. Low strength, with a minimum of 10 MPa, is often used for light building structural components. In contrast, high strength, with a minimum of 41 MPa, is employed for prestressed concrete and heavy building structural components. The experimental outcome will be categorised as the compressive strength of the samples.
The standard relates to concrete bricks and paving blocks whose strength is categorised into four categories for architectural components. The sample in this experiment followed a solid concrete brick between two types of concrete bricks: solid and hollow. Level I is the strongest, having a minimum strength of 10 MPa, and is commonly used for structural components such as bearing walls. The lowest level is level IV, which has a minimum strength of 2.5 MPa and is suitable for nonstructural components such as wall partitions. The highest level for paving blocks is A, which has a minimum strength of 40 MPa and is used for public roadways. The lowest level is D, which has a minimum strength of 10 MPa and is suitable for dwelling floors or garden pavement. Table 2 contains more information on architectural components and their applications.
Experimental procedure. During the experimental investigation, disposable diapers were substituted for fine aggregates to prepare composite material for construction components. The first step was to prepare used    Figure 1 depicts the aggregate test results, utilised as a base calculation of mix design for composite materials based on specific concrete and mortar mix design techniques. Following the aggregates test, Table 3 displays the mix of materials used to produce composite materials. It also discusses how the construction materials situation differed based on the structural and architectural components of the structure. Structural elements such as columns and beams pertain to the design of a concrete mixture, including Portland cement, fine and coarse aggregates, and water. In this experiment, the concrete mix design was initially formulated to achieve a maximum compressive strength of 25.00 MPa, which, according to Table 1, is typical for mid-rise structures. Refer to mortar mix design for architectural features such as walls and floors, where Portland cement, fine aggregates, and water are combined to form mortar. The composite material samples were then subdivided based on their intended use in construction components, such as concrete samples meant for structural components such as columns and beams with sample of cubes (dimensions: 15 × 15 × 15 cm) and cylinders (height 30 cm, diameter 15 cm). While examples of mortar blocks measuring 5 × 5 × 5 cm were developed for architectural components such as walls and floors, they were shaped to resemble mortar blocks. After 28 days of curing, the compressive strength of a sample of six samples was evaluated.
Determining low-cost housing standard. Since low-cost housing became a significant issue in this research, the housing standard for the implemented experimental study was designed by following standards of low-cost housing 63 . In this study, the housing is designed for four persons with a housing area of 36 m 2 and a land area of 60 m 2 . The floor plan of the design is shown in Fig. 2 (Table 4). Figure 3 shows the test on the compressive strength of concrete and mortar specimens, and the findings are displayed in Table 5. Regular concrete attains a strength of 24.91 MPa, close to the mix design's intended strength of 25 MPa. However, the substitution of fine aggregates with disposable diapers resulted in a weakening of the structure as the number of disposable diapers increased. Similarly, a common phenomenon happened in the mortar mix design. The strength decreases as more disposable diapers are substituted for fine aggregates.   www.nature.com/scientificreports/ Consequently, the strength value is plotted to the linear regression equations by estimating the effect of disposable diaper replacement on compressive strength and considering their use as construction materials. As seen in Fig. 4

, the employment of disposable diapers in concrete as a structural component is restricted by strength. The equation shown by linear regression is:
where y is the compressive strength, and x is the percentage substitution of fine aggregate by disposable diapers. Concrete's strength can be predicted using the equation. In addition, by referencing Table 1 of the relationship between the strength and use of concrete, Fig. 4 illustrates the use of disposable diaper concrete for housing construction.
As shown in Fig. 4, the use is restricted to structural housing with a maximum of three-story, and a total replacement is 10%. The replacement range between 0 and 10% can achieve a strength between 20 and 25 MPa. However, for structural components, the maximum replacement rate is restricted to 27%, with a maximum strength value is 10 MPa. The maximum replacement rate for nonstructural components is likewise advised to be at most 40%. More than this proportion, concrete cannot be utilised for construction materials.
The relationship between strength, percentage of replacement, and the use of mortar for architectural features is depicted in Figs. 5 and 6. In this study, the use is separated between concrete bricks and paver blocks based on Table 2 of strength standard and application as a construction material. In addition, the linear regression provided the equation for predicting the strength and percentage replacement of fine aggregates in mortar compounds: where y is the compressive strength, and x is the percentage substitution of fine aggregate by disposable diapers.
As seen in Fig. 5, the strength of concrete bricks is ranked from I to IV, with I being the strongest and IV being the weakest. The maximum amount of disposable diapers that can be substituted for fine aggregates to achieve the level I strength is 8%, and for total replacement is 40% to achieve level IV as the lowest standard of concrete bricks. The replacement rate is restricted to 40%, and additional replacement is not advised because it is out of SNI 03-0349-1989 60 . Then, as is seen in Fig. 5, the maximum replacement of fine aggregate by a disposable diaper for a structural element such as a bearing wall under an exposed situation is 8%, whereas, under a covered state, the maximum replacement is 19%. In addition, more than 19% replacement can only be applied for nonstructural parts, with a maximum of 33% replacement for covered situations and exposed finishes. In addition, with a maximum of 40% replacement for plastered covered circumstances. Over 40% of disposable diapers do not meet specifications and are unsuitable for use in concrete bricks.
Then, for paving blocks depicted in Fig. 6, the material is restricted to only the D level, with a maximum replacement rate is 9%. It found that more than 9% substitution of fine aggregates with disposable diapers is not allowed for paving blocks due to the strength being below SNI 03-0691-1996 specifications 61 . Additionally, 9% of replacement is restricted to paving the floors of homes and gardens.
Finally, to figure out comprehensive findings, the usage of disposable diapers on composite materials for the building materials is depicted in Fig. 7, which shows the various application of materials depending on their strength and component. In general, materials for concrete as structural components, such as columns and beams, can be substituted with disposable diapers to a maximum extent of 27% and a maximum strength of 10 MPa. For www.nature.com/scientificreports/ mortar as a structural component, such as bearing walls and public road pavement, the maximum replacement by a disposable diaper is between 8 and 9% with a strength of 8.5 MPa. Alternately, for maximal replacement, it can be used for nonstructural components with a maximum of 40% replacement and strength of 2 MPa. This application is for non-load-bearing wall partitions and low-impact floor pavers.
In addition, the result of the experimental study is applied to design requirements for low-cost housing based on Table 4 and Fig. 2. The use of composite materials in the structural and architectural components of a 36 m 2 housing design is depicted in Fig. 8, with a maximum percentage of disposable diapers for housing components. For example, the maximum percentage of disposable diapers for column and beam structural components is 27%, with a strength of 10 MPa. The maximum percentage of disposable diapers for walls and floors is 40% and 9%, respectively, with a strength of 2 MPa and 8.5 MPa.  www.nature.com/scientificreports/ However, the structural analysis was not calculated exhaustively for this design scenario's column and beam size-the measurement related solely to the standard dimension of structural components for one-story housing in Indonesia. Further research and implementation of these findings must focus on more extensive structural assessments, including soil-bearing capability, load capacity, and other technical tests for structural analysis. Ultimately, to quantify the amount of composite material for the housing design, the volume of each construction component is determined using the floor design, and also accessible is the number of disposable diapers. The outcome is displayed in Table 6.
According to Table 6, the total amount of construction materials required to build housing type 36 is 22.79 m 3 with 1.73 m 3 of disposable diapers. It indicates that a maximum of 7.6% of disposable diapers can substitute fine aggregate in construction. This finding gives insight into the effectiveness of the materials to be applied as building components in architectural design and further research. Also, by considering the environmental value of waste recycling, the material gives benefit to be developed on a large scale and by involving society and other stakeholders in collecting and managing the waste of disposable diapers. Study implication. Currently, the essential step in the recycling process for used diapers is to separate the plastic components from the organic fibres. It necessitates the execution of many complicated procedures, including collecting, crushing, sanitising, and sorting the components. Due to the difficulty involved in the process, only a few businesses are currently interested in recycling used diapers, such as Knowaste Ltd 69 . However, the existence of the companies reveals that diaper recycling technology is currently only available in developed countries. It is primarily the result of two factors: differences in levels of expertise and access to equipment between developed and developing countries and a need for more awareness in developing countries regarding the potentially harmful effects of diaper waste 70 .  www.nature.com/scientificreports/ It becomes easily applicable by combining the waste as a part of composite materials such as concrete or mortar. Concrete is a widely used construction material due to its ease of processing, relatively low costs, and lack of high-tech manufacturing requirements. This research has concluded that adding used diapers to concrete does not significantly diminish its strength. It demonstrates that using used diapers to create composite materials is feasible, particularly concerning the development of environmentally friendly and cost-effective materials. Further, concerning this paper's social and economic advantages, the development of materials can be accessed from low to high technology. The procedures are relatively easy to conduct and low-cost. It also gives a comprehensive perspective of utilising disposable diaper waste as something valuable since it has ended up in the incineration process.
However, there are several limitations to implementing the findings broadly. To address the materials in wider applicable and in massive utilising, it needs the involving of stakeholders for waste treatment such as collecting the waste from households and washing the diaper waste until sanitising. The need for machines to shred the waste is also crucial to produce on a large scale due to low technology only being able to approach small-scale materials production. In addition, due to the existing various building rules and regulations only limited to conventional building materials, the role of government in regulating such materials needs to be opened.
At the same time, the limitations also give other benefits for future studies. The involvement of stakeholders and waste treatment mechanisms need to explore more to fill the gap. The innovation of shred machines for such materials can be challenging to be solved and invent. Moreover, to be implemented as low-cost housing, the materials need to be evaluated in terms of technical construction, cost, and housing price. This evaluation is proof to propose the materials in the financial mechanism of housing.

Conclusion
The conclusion is drawn that using disposable diapers on composite building materials is represented by the linear regression equation y = − 56.681x + 26.191 for concrete and y = − 20.57x + 10.364 for concrete mortar. Where y is referred to compressive strength, and x is referred to the percentage replacement of fine aggregate by the disposable diaper. The utilisation is then divided into building components such as concrete utilisation covers column and beam with the maximum disposable diaper is 10% can obtain the strength of 20 MPa. This strength is appropriate for a three-story house. In contrast, a maximum utilisation of 27% is recommended for a single floor of housing with a strength of 10 MPa. The greater the replacement, the lower the SNI structural standard, and only recommended for nonstructural components.
In addition, in applying the composite mortar to wall and floor elements comprised of concrete bricks and paver blocks, respectively. The full replacement for a wall is 40%, resulting in 2 MPa of strength and classification as level IV concrete bricks, according to SNI 03-0349-1989. A full replacement of 9% is required for floor paving blocks, resulting in a strength of 8.5 MPa and compliance with level D of the SNI 03-0691-1996 standard for paving blocks. This maximum quantity is suitable for partition walls with no load-bearing capacity and floor pavers with low impact capacity. Replacement of more than this maximum capacity is not recommended for the utilisation of building materials. Incorporating experimental study findings into the design of low-cost housing, the total waste capacity of disposable diapers that can be utilised for single-story housing type 36 (36 m 2 ) is 1.73 m 3 out of a total composite material volume of 22.79 m 3 .

Data availability
The datasets generated during and analysed during the current study are not publicly available due to the patent process (Pat. Pend. No. P00202213376) but are available from the corresponding author upon reasonable request.